These are custom made by someone near me for audiophile purposes. DC blockers are not uncommon audiophile components. PS Audio produced the Hum Buster II until no so long ago. Likewise for the Channel Islands Audio AC Filter. DC contamination of AC power lines is a common problem, specially for people living in buildings with multiple apartment units in them. DC contamination will cause hum in components' transformers. Depending on your equipment and configuration, this hum may be loud enough to be audible from the listening position and even be amplified and come out of the speakers. Hum can seriously degrade sound quality. The transformer's vibration may also degrade the resolution and soundstage of your system.
I have a noise problem in my apartment and it was suggested to me that it was DC contamination, so I had one made as both PS Audio and CI Audio units are discontinued. However, in my particular case, the noise is not DC contamination and the hum did not improve with DC filtering. I was able to later confirm this with an electrician. However, I had ordered this filter with really high-quality components. It cost quite a bit more than I am asking for it. It is just a shame to keep it around collecting dust, when someone may actually have good use for it.
